1. Field of the Invention
The present invention pertains to surgical cutting instruments and, more particularly, to surgical cutting instruments having elongate inner and outer tubular members with distal ends cooperating to cut or resect bodily tissue, the cut tissue being aspirated through the inner member.
2. Discussion of the Prior Art
The use of elongate surgical cutting instruments has become well accepted in performing closed surgery, such as arthroscopic or, more generally, endoscopic surgery. In closed surgery, access to the surgical site is gained via one or more portals, and instruments used in the surgical procedure must be elongated to permit the distal ends of the instruments to reach the surgical site. Surgical cutting instruments for use in closed surgery conventionally have an elongate outer tubular member terminating at a distal end having an opening in the side wall, the end wall or both to form a cutting port or window and an elongate inner tubular member concentrically disposed in the outer tubular member and having a distal end disposed adjacent the opening in the distal end of the outer tubular member. The distal end of the inner tubular member has a surface or edge for engaging tissue via the opening in the distal end of the outer tubular member and in many cases cooperates with the opening to shear or cut tissue. The inner tubular member is rotatably driven at its proximal end, normally via a handpiece having a small electric motor therein controlled by finger-actuated switches on the handpiece, a foot switch or switches on a console supplying power to the handpiece. The distal end of the inner tubular member can have various configurations dependent upon the surgical procedure to be performed, and the opening in the distal end of the outer tubular member has a configuration to cooperate with the particular configuration of the distal end of the inner tubular member. For example, the inner and outer tubular members can be configured to produce whisker cutting, synovial resection, arthroplasty burring or abrading, side cutting, meniscus cutting, trimming, full radius resection, end cutting and the like, and the various configurations are referred to generically as "cutting blades or edges". Cut tissue is aspirated through the hollow lumen of the inner tubular member to be collected via a tube communicating with the handpiece.
It is very important in such surgical cutting instruments that the cutting edge be precisely positioned and aligned relative to the opening in the distal end of the outer tubular member. Accordingly, a bearing structure must be provided to obtain precise positioning and alignment of the cutting edge while permitting unrestricted rotation of the inner tubular member within the outer tubular member. That is, the inner tubular member cannot be allowed to cock or be positioned askew of the longitudinal axis of the surgical cutting instrument as a reaction to the cutting action. This problem has been difficult to overcome in the past. The addition of bearing surfaces at the distal end of the surgical cutting instrument or at spaced positions along the surgical cutting instrument requires complex structure and manufacturing techniques and results in a relatively expensive product that cannot be feasibly supplied for single patient use, i.e., be disposable. Additionally, such bearing structures present increased opportunities for malfunction due to sticking and obstruction.
Another manner in which to precisely position and align the inner member is to make the outer diameter of the inner member substantially the same as the inner diameter of the outer tubular member, or as close as possible thereto, so that there is little or virtually no gap or clearance between the inner tubular member and the outer tubular member. However, it is preferred to construct the inner tubular member and the outer tubular member of stainless steel, and the physical properties of stainless steel limit its effectiveness as a rotational bearing surface. In particular, stainless steel is relatively soft, having a surface hardness of 30-35 Rc, and possesses a high coefficient of friction. Moreover, the inner and outer stainless steel tubular members are subjected to considerable thrust and radial loads during operation wherein the inner tubular member can rotate at speeds greater than 2,500 RPM. If the stainless steel surfaces of the inner and outer tubular members bear on and contact each other, such as occurs when the clearance between the inner and outer member is small, heat will be generated from friction and cause thermal expansion of the inner tubular member. As the area of contact between the inner tubular member and the outer tubular member increases, greater friction will be created with a concomitant increase in heat eventually causing the grain structure of the stainless steel tubular members to "flake", in turn increasing temperature due to abrasion until the inner tubular member has expanded further than the gap between it and the outer tubular member and the instrument seizes. No adequate solution to this problem has been found prior to the present invention, it being noted that biocombatibility must be considered in using a lubricant coating between the inner and outer members. Thus, prior art efforts have sacrificed optimum cutting quality for prolonged life of the cutting instrument by increasing the clearance between the inner tubular member and the outer tubular member. A typical range for the clearance maintained between the inner tubular member and the outer tubular member of a surgical cutting instrument ranges from a minimum of 0.00115 inches to a maximum of 0.00285 inches. The clearance associated with conventional surgical cutting instruments results in reduced cutting efficiency, and there is a need to produce precision cutting with such instruments to allow their use in various different surgical procedures.
Titanium nitride (TiN) has been used on cutting tools for industrial applications to provide the benefits of longer life and higher productivity as described, for example, in various literature and brochures from Balzers Tool Coatings, Inc. TiN coatings are applied by placing physically and chemically clean tools in fixtures to become the cathode of a high voltage circuit in a reaction chamber that is evacuated and charged with argon. By sputter cleaning, positive argon ions are propelled by a high voltage field and blast the tool to make the tool atomically clean. An electron beam gun heats titanium until the titanium evaporates. Nitrogen is introduced into the chamber, and the titanium ions are electrically accelerated toward the tools. The titanium ion bombardment combines with the nitrogen gas to form a coating of TiN about 0.0001 inch thick on the surface of the tool. The coating process is called "physical vapor deposition" and operates at temperatures in the 900.degree. F. range.
In the past, cutting tools have been coated at the cutting edges thereof with TiN by physical vapor deposition for the purpose of hardening the cutting edge thereby providing the cutting tool with an extended useful life; however, there has been no recognition of the use of TiN coatings by physical vapor deposition to limit heat conduction along a rotating tubular member to produce a bearing structure for inner and outer members made of stainless steel.
A further consideration affecting the performance of a cutting instrument including an inner tubular member movably mounted within an outer tubular member relates to the cutting aperture configuration. As previously noted, any number of configurations may be formed in the distal end of the inner tubular member depending upon the surgical procedure to be performed, and the window or port in the distal end of the outer tubular member is configured to cooperate with the particular configuration of the distal end of the inner tubular member. The interface between the stationary outer tubular member and the movable inner tubular member governs the sharpness of the cut and the efficiency of the cut as determined by the closing force required to complete the cut. Traditionally, the cutting surface or edge in the distal end of the inner tubular member is formed by cutting away a portion of the inner tubular member above its center line. The resulting cutting edge or surface lies above the center line of the inner tubular member and thus includes a relatively dull obtuse angle on the outside diameter of the inner tubular member requiring significant closing force to move the cutting edge past the cutting port or window in the distal end of the outer tubular member when shearing tissue.